The present invention relates to the field of cutting tools and particularly to coatings for ceramic coated hard metal cutting tool inserts used for cutting, milling, drilling and other applications such as boring, trepanning, threading and grooving.
Coatings improve the performance of cutting tools, especially ceramic or oxide coatings on carbide or hard metal cutting tools. Ever since carbide cutting tool inserts have been ceramic coated with, for example, aluminum oxide (Al2O3), there has been a continuing effort to improve the adherence of the coating to the substrate. When the first aluminum oxide coating was applied directly to a substrate of the carbide or hard metal type, the oxygen in the aluminum oxide reacted with the substrate which reduced the adherence.
It has been known to improve the properties of tool inserts made from a sintered hard metal substrate (metallic carbide bonded with a binder metal) by applying a wear-resistant carbide layer. See UK Patent Nos. 1,291,387 and 1,291,388 which disclose methods of applying a carbide coating with improved adherence; specifically, controlling the composition of the gas used for deposition of the carbide so that a decarburized zone was formed in the sintered hard metal at the interface with the wear-resistant carbide. The decarburized zone known as an eta layer, however, tends to be hard and brittle, resulting in breakage. It has also been known to apply a ceramic or oxide wear-resistant coating (usually aluminum oxide) upon the sintered metal substrate. However, as already explained, the oxide layer directly upon the sintered metal body may disrupt the sintered metal morphology and binding ability. A number of patents have disclosed the use of an intermediate layer of carbides, carbonitrides and/or nitrides. See U.S. Pat. Nos. 4,399,168 and 4,619,866. An intermediate titanium carbide (TiC) layer improved toughness but still an eta layer existed limiting the application of the coated tool inserts to finishing cuts. A layer of titanium nitride (TiN) applied before the TiC layer eliminated the eta layer but toughness was still less than required. See U.S. Pat. No. 4,497,874. Intermediate layers of titanium carbonitride (TiCN) in place of the TiC intermediate layer have been proposed. See U.S. Pat. Nos. 4,619,866 and 4,399,168. A thin surface oxidized bonding layer comprising a carbide or oxycarbide of at least one of tantalum, niobium and vanadium between the hard metal substrate and the outer oxide wear layer has been proposed. See U.S. Pat. No. 4,490,191.
The ceramic coating (Al2O3) does not adhere well enough to the TiC and many TiCN intermediate coatings when used to enhance the adhesion of the coating to the cemented carbide substrate. Due to thermal expansion differences, there is a tendency to delaminate. With the stress caused by the thermal expansion difference, coatings tend to perform inconsistently. These intermediate coatings are mostly characterized by a straight line interface between the intermediate coating and the oxide coating as shown in FIG. 1. This results in a weak bond. Adhesion may be increased some by making the substrate rough but the projections provided by the roughening are spaced too far apart to perform consistently.
Another problem experienced with carbide and hard metal cutting tools is the frequent failure of those tools due to thermal shock. The inserts become very hot during cutting and then cool upon application of coolants or when disposed outside the cut. Cycles of heating and cooling result in steep temperature gradients within the inserts, and the accompanying stresses may cause cracks in the inserts that initiate fractures and reduce tool life. Thus, coatings that reduce the occurrence of fractures from thermal shock may considerably enhance tool life.
With the coatings, according to the present invention, increased wear resistance as well as adhesion strength are provided in ceramic coatings on hard metal cutting tools. According to another aspect of the invention, coatings are provided that reduce thermal shock experienced by carbide and hard metal cutting tool inserts.
Briefly, according to this invention, there is provided a cutting tool insert comprising a hard metal substrate having at least two wear-resistant coatings. One of the coatings is a ceramic coating. An intermediate coating under the ceramic coating is comprised of carbonitride having a nitrogen to carbon atomic ratio between about 0.7 and about 0.95 whereby the carbonitride coating forms fingers interlocking the ceramic coating, thus improving the adherence and fatigue strength of the ceramic coating. Preferably, the nitrogen to carbon atomic ratio in the carbonitride coating lies between about 0.75 and 0.95 as determined by X-ray diffraction. The cutting tool insert also may include an additional coating, deposited on the substrate, that is a layer of at least about 2 microns, and preferably at least about 2 up to about 5 microns, in thickness and comprises at least one of a metal carbide, a metal nitride, or a metal carbonitride of a metal selected from zirconium and hafnium.
According to one embodiment of this invention, the hard metal cutting tool insert has two intermediate coatings between the hard metal substrate and the aluminum oxide surface coating. The coating adjacent the substrate is a 1 to 4 micron layer of titanium nitride. The coating over the titanium nitride layer is a 2 to 4 micron thick titanium carbonitride layer and the aluminum oxide coating is a 1 to 10 micron layer.
According to the preferred embodiment, the hard metal substrate of the cutting tool insert has four coatings as follows: a 2 micron titanium nitride interior coating, a 3 micron titanium carbonitride intermediate coating, a 6 micron aluminum oxide intermediate coating, and a 2 micron Ti (C, N), i.e., TiC, TiN, TiCxNy exterior coating.
Titanium is not the only suitable metal for use in the carbonitride coating. The metal may be comprised of, in addition to titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten.
The cutting tool insert substrate, according to this invention, typically comprises 3% to 30% of a binder metal from the iron group including, in addition to iron, nickel and cobalt and mixtures thereof and between 70% and 97% of a carbide selected from the group tungsten carbide, titanium carbide, tantalum carbide, niobium carbide, molybdenum carbide, zirconium carbide and hafnium carbide. In addition to carbides, the cutting tool insert substrate may also include nitrides.
According to a preferred embodiment, the cutting tool insert substrate has a binder phase enriched surface layer, that is, a surface layer enriched with a higher percentage of cobalt or other binder metal.
Briefly, according to this invention, there is provided a method of making a coated cutting tool insert having a wear-resistant coating comprising the steps of depositing a metal carbonitride coating having a nitrogen to carbon atomic ratio between about 0.7 and about 0.95 by adjusting the reactants used for chemical vapor deposition of said coating and depositing a ceramic coating directly over said carbonitride coating whereby said carbonitride coating and ceramic coating have interlocking microscopic fingers.
According to another aspect of the invention, there is provided a cutting tool insert including a hard substrate and a plurality of coatings on at least a portion of the substrate. The substrate may be any type suitable for use as a cutting tool insert and may be, for example, a cemented carbide as described above. The plurality of coatings includes at least a first and a second coating. The first coating is a layer at least about 2 microns and preferably about 2 to about 5 microns in thickness deposited on the substrate and includes at least one a metal carbide, a metal nitride, or a metal carbonitride of a metal selected from zirconium and hafnium. Preferably, the first coating is a layer of zirconium nitride or hafnium nitride. The second coating is a layer including at least one of a metal carbide, a metal nitride, or a metal oxide of a metal selected from groups IIIA (B, Al, Ga), IVB (Ti, Zr, Hf), VB (V, Nb, Ta), and VIB (Cr, Mo, W) of the periodic table. One or more additional layers optionally may be provided intermediate the first and second coatings and also may be deposited exterior to the second coating. Thus, for example, the plurality of coatings may include a reinforcing coating, as described herein, provided intermediate the first and second coatings. The intermediate coating contacts and enhances adhesion of the second coating. More particularly, the intermediate coating may be a layer including a metal carbonitride that, as described herein, has a nitrogen to carbon atomic ratio that results in superior adherence of the second coating due to the development of interlocking fingers between the second coating and the intermediate coating.
Designations such as xe2x80x9cfirstxe2x80x9d, xe2x80x9csecondxe2x80x9d, and xe2x80x9cthirdxe2x80x9d are used herein to identify individual coatings or layers only and, in the present description and the attached claims, do not necessarily refer to the ordering of the layers or coatings or their sequence of application on the substrate. Thus, for example, a xe2x80x9cfirstxe2x80x9d coating or layer is not necessarily in contact with or immediately adjacent a xe2x80x9csecondxe2x80x9d coating or layer, and a xe2x80x9cthirdxe2x80x9d coating or layer, as well as additional coatings or layers, may be deposited intermediate the xe2x80x9cfirstxe2x80x9d and xe2x80x9csecondxe2x80x9d coatings or layers.